In recent years, next-generation mobile communication systems have been actively studied and a one-frequency repeating cellular system in which individual cells use the same frequency band has been proposed as a scheme for improving efficiency of frequency utilization in such systems.
For downlink (i.e., communication from a base station apparatus to a mobile station), Orthogonal Frequency Division Multiple Access (OFDMA) is the most promising candidate. OFDMA is a system that performs communication by allocating slots, which are units of access consisting of time axis and frequency axis, among multiple mobile terminal apparatuses and using OFDM signals which are produced by modulating information data such as by 64-ary Quadrature Amplitude Modulation (64 QAM) or Binary Phase Shift Keying (BPSK). Because it uses OFDM signals, Peak to Average Power Ratio or PAPR can become very high. High peak power is not a very large problem in downlink communication which has a relatively large margin in transmission power amplification capability, but can be a significant issue in uplink (i.e., communication from a mobile station to the base station apparatus) which has no margin in transmission power amplification capability.
Also, to reduce effect of interference occurring in one-cell repetition, a CDM-OFDM scheme has been studied that distributes one piece of data across multiple carriers (this process is called “spreading”) and sends multiple pieces of data multiplexed (i.e., spreads data using orthogonal codes for multiplexing). It is said that in generation of CDM-OFDM signals, use of orthogonal codes enables data to be demultiplexed, ideally on a receiver, and use of scrambling codes specific to the base station can spread interference as well.
Non-Patent Document 1 listed below discloses CDM-OFDM (referred to as “OFCDM” in the document). According to the document, in an isolated cell, better throughput characteristics are obtained when spreading in frequency domain is not used because such a cell is little affected by interference, whereas in an environment largely affected by interference, e.g., in a one-frequency repeating system, spreading in frequency domain results in better characteristics.
Meanwhile, Patent Document 1 shown below proposes a scheme for switching an access method. The scheme shown in the document switches between multi-carrier and single-carrier. The scheme is basically intended for use in uplink and proposes that single-carrier scheme be selected when high transmission power is required and the OFDM scheme when low transmission power is required.
Non-Patent Document 1: IEICE TRANS. COMMUN., VOL. E86-B, NO. 1 January 2003 “Variable spreading-factor-Orthogonal Frequency and Code Division Multiplexing (VSF-OFCDM) for Broadband Packet Wireless Access”
Patent Document 1: JP Patent Publication (Kokai) No. 2007-151059 A